The present invention relates generally to computing systems with bus-connected peripheral devices, and more particularly to systems having bus architectures that require termination.
An important and valuable part of any computing system today is its external storage facility. A presently preferred form of such storage is some type of hard disk structure because of its random access capability although other forms (e.g., tape, CD-ROM) are also used. This type of storage, often referred to as mass storage, is used to amass huge amounts of data for quick, easy access. There are a number of bus architectures capable of connecting a processor unit to such mass storage devices. One that is very flexible and powerful, particularly for connecting mass storage devices, and other peripheral elements, is the small computer system interface (SCSI).
The physical SCSI bus structure comprises a set of conductive signal lines that carry data and control signals for information transfer between the devices on the bus. Signals and data are communicated according to pre-determined protocols. (See, for example, Working Document for Information Technology SCSI Parallel Interconnect 2 (SPI-2), ANSI X3.302.)
A well-known physical characteristic of SCSI bus structures, shared by a number of other bus architectures, is that the signal lines forming the bus are terminated by an SCSI bus terminator at each end of the bus in order to reduce signal reflections of the bus that can degrade the information signals communicated thereon. Typically, the device or element connected at the bus"" end includes the needed termination. Problems arise, however, when the device with the termination is removed from the bus, or the device is shut down. Removal/disconnection of the device with the terminator removes that termination, and often results in degradation of the signals communicated on the bus so that the entire system is effectively brought down. In order to continue operation, the termination must be moved to the next in line last device on the bus.
This problem can also occur if the connection of the last device is inadvertently disconnected, the entire system will again be affected because the communicating ability of the SCSI bus is lost. This is a problem that is not necessarily indigenous to SCSI bus architectures, but can be encountered by any bus architecture requiring some form of termination at the two ends of the bus to prevent degradation of the signals communicated thereon.
The present invention provides a method, and apparatus for implementing that method, for permitting failure or removal of that processing device that terminates the bus to which it is communicatively coupled. On-line service of a failed element that carries the termination of a bus to which the element connects is provided.
The present invention finds applicability in a processing system includes a number of processing devices coupled to a system bus for data communication therebetween. Those processing devices (xe2x80x9cterminating devicesxe2x80x9d) at each of the ends of the bus have the circuitry necessary for electronically terminating the system bus in order to minimize the reflections and other phenomena that can occur of such a bus as a result of high-speed data transmission. According to the present invention, one of the processing devices (other than the terminating devices) includes a termination adapter circuit. In the event the one of the terminating devices is removed or looses power, the termination adapter circuit will sense that removal or loss of power and automatically disconnect the bus connection to the failed or removed terminating device, and terminate the bus with a termination that replaces that lost with the failed or removed terminating device. The processing device having the termination adapter circuit now becomes the new terminating device, providing the requisite bus termination.
The invention is described in the context of a computing system in which processor units are coupled to peripheral devices (e.g., storage units) by a SCSI bus architecture. However, it will be apparent to those skilled in this art that the invention is applicable to any bus requiring termination. Conventionally, a SCSI bus structure includes, among the signal lines implementing that structure, a signal line carrying the signal xe2x80x9cTermination Powerxe2x80x9d (TERM PWR). TERM PWR is a 5 volt signal that supplies current to the terminators at the end of the SCSI bus. In a preferred embodiment of the invention, TERM PWR is monitored by a termination adapter circuit located in association with a processing device coupled to the SCSI bus. Should the terminating device lose power, or the bus connection to the terminating device is interrupted, the TERM PWR signal will be lost. Loss of TERM PWR will be noted by the termination adapter circuit, causing it to switch in a new SCSI bus terminator. At the same time, the termination adapter circuit will electronically disconnect the bus from the failed or lost terminating device. The processing device associated with the termination adapter becomes the new termination device and thereby, the SCSI bus for data transfers remains.
The implementation is symmetric; the termination adaptor is located in each of those devices immediately proximate the next in line from the last devices on the SCSI bus. Therefore, the loss of a device on either end of the SCSI bus follows a similar procedure.
A number of advantages are achieved by the present invention. First, according to one embodiment of the invention, a system having a number of processing devices coupled to a bus having terminations at the ends of the bus can lose one termination (i.e., the terminating device having the termination) and continue to operate as before. Thus, the terminating device can be removed from the bus, or shut down for repair, without having to shut down the entire system connected to the bus even temporarily. The remaining processing elements can continue to operate.
In a further embodiment of the invention, both terminating devices to the bus can be removed or otherwise lost, yet the remaining devices can still communicate with one another.
Further, should the bus cable that connects to the terminating device be even inadvertently disconnected, the entire system is not caused to fail.
This invention is particularly useful in non-stop computer systems that require repair while the rest of the system continues to operate.
These and other advantages and aspects of the invention will become apparent to those skilled in the art upon a reading of the following detailed description of the invention, which should be taken in conjunction with the accompanying drawings.